Abscess formation is a classic host response to bacteria during sepsis. Certain bacterial species, particularly Bacteroides fragilis and Staphylococcus aureus are predisposed to induce abscesses. The essential bacterial virulence factor required for abscess induction by these two pathogens is a capsular polysaccharide with a zwitterionic charge motif. Zwitterionic polysaccharides (ZPS) induce the host to form abscesses by their ability to activate T cells initiating a proinflammatory Th1 cytokine response. In contrast to the immunologic paradigm defining polysaccharides as T cell independent antigens, ZPS activate T cells in vitro as well as in vivo when incubated with antigen presenting cells (APC). There is currently no immunologic model that describes how purified polysaccharides can activate T cells. ZPS are internalized and can be detected in lysates of intracellular vesicles from the APC. Blocking of endosomal acidification results in the failure of ZPS to activate T cells. ZPS recovered from endosomal vesicles has a substantially reduced molecular size, indicating processing. We have demonstrated that MHC class II DR appears to be the molecule used by the APC to present ZPS to the T cell and that TCR alpha beta( is required for T cell activation. We hypothesize that ZPS are internalized and cycle through the APC, and that this process is required for presentation of the ZPS to the T cell. We intend to define a novel immunologic paradigm that describes how an important class of biologic molecules (carbohydrates) is recognized by the cell-mediated immune system. This will be done by investigating the cellular pathway by which ZPS cycle through the APC and activate CD4+ T cells. We have defined four specific aims: 1) Determine how ZPS are altered within the endocytic pathway and define the molecular requirements for ZPS-mediated T-cell activation; 2) Investigate the vesicular trafficking and intracellular interactions of ZPSs in the endocytic pathway; 3) Characterize the binding interactions of the MHC class II DR molecule with ZPS; 4) Determine whether T-cell activation results from "processed antigen" presentation or superantigen presentation and whether the processing of ZPS uses the same pathway as protein processing. The delineation of a mechanism for carbohydrate processing and presentation has broad relevance to the fields of microbiology and immunology and could lead to new concepts for enhancing T cell recognition of other polysaccharides.